Edit:
In terms of standards compliance, Opera is the only browser that can (closely) accurately render and pass the Acid2 test set up by the Web Standards Project:
http://www.webstandards.org/action/acid2/
http://www.webstandards.org/2006/07/20/acid2-and-opera-9-clarifications/
It is the most standards-compliant browser as it follows the W3C guidelines to a T.
These are reasons that developers should embrace because IE's atrocious lack of standards compliance vs its browser domination means extra work for everyone who creates web applications because they are forced to write hacks and exception codes for IE. Therefore:
Opera > Firefox > IE > Safari
In terms of features, you can't beat Firefox with its extensive library of plugins that extend the functionalities of your browser (you can turn it into an FTP client, for example).
Come on. Safari for Windows (and probably Mac as well when pitted against Opera or Firefox) a joke by modern browser standards:
http://www.extremetech.com/article2/0,1558,1990850,00.asp
http://www.extremetech.com/article2/0,1697,2152777,00.asp
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